This invention relates to reclosable plastic bags and an apparatus used to integrally extrude closure elements with a plastic film so that the film can be formed into reclosable bags. More particularly, the invention relates to an apparatus used to integrally extrude closure elements with films of varying thickness and other structures.
Plastic films formed into bags having reclosable closure elements located at the top of the bag are well known. Two basically different approaches are used to manufacture films having these closure elements. In the first approach, the closure elements are extruded as strips and are later attached to the film by heat or by other means of adhesion. This approach, however, requires a separate inventory of the film and the closure elements, together with additional equipment to adhere the elements to the film.
The second approach is to extrude the elements simultaneously with the film. In this approach, either a blown film process or a cast die process is used. Typically, in the blown film process, the film and closure elements are extruded from a single annular die head to form a tubular film. One difficulty with the blown film process is maintaining certain critical features of the closure elements. Because the film can be stretched longitudinally and laterally, the closure elements are subject to distortion.
In the cast film die process, of which U.S. Pat. No. 4,515,647 is an example, the resin or plastic extrudate is injected into a die, typically of the "coat hanger" type, which is well known to those in the art. Within the die, the extrudate spreads laterally across the die into a uniform stream where it exits the die lip as a thin film. Where the closure elements are formed simultaneously with the film, the same plastic extrudate used to form the film may also be used to form the closure elements. A disadvantage of this method is that the position of the profiles for the closure elements with respect to the mainstream film cannot be adjusted.
Alternatively, separate die blocks for the closure elements may be used, as in U.S. Pat. No. 4,263,079. In this arrangement, the closure elements are joined to the film immediately after each element exits its respective die lip and while it is still in a fusible state. A disadvantage with this method is that separate and additional equipment is necessary.
The present invention provides for extruding the closure elements integrally with the plastic film using a cast film die having a removable lower die lip with an upper surface having one or more slots in the shape of closure elements. Additionally, the die is provided with an adjustable choker bar so that films of varying thicknesses having closure elements in variable positions can be produced.